The present invention relates to image processing methods and image processing apparatus.
Image processing apparatus that acquires image information from an image input unit such as a digital still camera, flat-bet (FB) scanner, or film scanner, then provides the image information with various types of image processing, and prints images onto an image output unit such as an ink jet printer, thermal sublimating printer, or silver salt digital printer for obtaining images by providing silver salt photographic printing paper with a scanning exposure process and a developing process by use of an array-type exposure head or a laser, and equipment by which the images that have been acquired from the above-mentioned image input units are further stored as image information onto image recording media such as a CDR, have come into most common use in recent years.
These pieces of equipment are applied not only to reproduce read images by printing them as they are, but also to combined use of various types of digital image processing such as providing spatial filter processing for the adjustment of sharpness, providing color conversion to obtain clearer photographs, and/or providing specific color tone conversion for the purpose of, for example, correcting the contrast of the photographs which have suffered defects in contrast reproducibility due to under-exposure (mistakes in photographing).
In general, however, image information that has been acquired from image input units contains noise, whether it be varying in degree. Such noise has the characteristic that when spatial filtering is provided in an attempt to improve sharpness or color conversion is provided in an attempt to obtain clearer photographs, the noise level will be augmented and conspicuous.
Incidentally, as will be detailed later, the spatial-filtering processing (hereinafter, also referred to as a spatial filtering) includes various kinds of operations, such as an operation for finding sum-of-product values between noticed pixels and peripheral pixels, an operation for obtaining information in regard to the spatial frequency and the amplitude by employing Fourier transformation processing to adjust the amplitude at every spatial frequency, etc. The object of such the spatial-filtering processing is to change sharpness, resolution, granularity and spatial-frequency response characteristics of the source image.
If, during photographing with a color negative film, the scene is underexposed for whatever reason and provided with a great deal of color tone conversion (the improvement of shadows in contrast) in an attempt to correct this underexposed scene that has decreased in the contrast of shadows due to the under-exposure, the noise components will also be enlarged as they are, and then if the film is spatially filtered to improve sharpness, the noise itself will also be increased. In addition, for the type of printing system that obtains images by providing a silver salt photosensitive material with digital exposure or sublimating and/or transferring a thermal sublimating color material, the images finished will usually be slightly blurred, which will result in deteriorated contrast.
If correcting these blurred portions of the images is attempted, the noise level will be further increased. Image blurs usually occur in relatively large areas, and if improving this state is attempted, the calculation time required for the improvement will also increase to an unacceptable level.
Also, in order to obtain a photograph of the desired size, image information is usually enlarged or reduced in size according to the particular image resolving power of the image processing apparatus. Such an enlarging/reducing process mostly uses a linear interpolation method in terms of factors such as processing speed and the preferability of image processing. In such a case, there is the problem that if the noise contained in the image increases above a certain level or if sharpness enhancement is strengthened to obtain sufficient sharpness, when the image undergoes processing at a magnification close to its original scaling coefficient (for example, when the image is enlarged at a scaling coefficient of 1.05 using a 300-dpi output unit), the noise level of the image will change cyclically (in the above example, at cycles of 1/15 of an inch) and the noise will appear as a moire-like fringe pattern.
Therefore, various techniques for reducing or removing the noise components of images are being studied in order to solve the above-described problems associated with image quality. However, the use of a noise reduction filter by which a sufficient reduction or removal effect can be obtained usually presents other problems such as the deterioration in the details of the image. Also, since noise filtering, as with spatial filtering, requires a long processing time, serious problems arise in terms of equipment costs, operational convenience, and operating capabilities.